Gear pumps, such as gerotor pumps, are well known and have been widely employed in a variety of applications for a number of years. Such pumps are positive displacement pumps wherein a rotor set, comprising an inner rotor having a given number of teeth N and an outer rotor having at least N+1 teeth, is rotated to pressurize a working fluid.
The center of rotation of the inner rotor of the rotor set is located eccentrically to the center of rotation of the outer rotor of the rotor set such that, as the rotor set is driven, a series of variable volume pumping chambers are formed between the teeth of the inner rotor and outer rotor. As the volume of a pumping chamber begins to increase, that pumping chamber enters into fluid communication with the inlet port of the pump so that low pressure working fluid is drawn into the pumping chamber. As the rotor set continues to rotate, the volume of the pumping chamber reaches its maximum and the chamber moves such that it is no longer in fluid communication with the inlet port resulting in the pressurization of the working fluid. As the rotor set continues to further rotate, the volume of the pumping chamber begins to reduce and the pumping chamber enters into fluid communication with the outlet port of the pump. As the volume of the pumping chamber continues to reduce, the working fluid therein is expressed into the outlet port and then into the pump outlet.
While such pumps are widely employed, they do suffer from problems. In particular, it has proven difficult to fill the pumping chamber from the pump inlet when the inlet pressure is low and/or when the operating speed of the pump is high and such difficulties can result in cavitation and increased operating noise. Most early approaches to improving filling of the pumping chambers comprised attempts to provide inlet ports of the largest practical size. However, the results obtained from such designs where less than satisfactory in many applications for a variety of reasons.
U.S. Pat. No. 4,836,760 to MacLeod teaches another approach to enhancing the filling of pumping chambers wherein the inlet port is located radially inward of the outer diameter of the pumping chambers. MacLeod recognized that, due to the centrifugal forces developed by rotation of the rotor set, the working fluid in the pumping chambers experiences a pressure gradient with the fluid adjacent the outer diameter of the rotor set being at the highest pressure. By moving the inlet port radially inward, MacLeod teaches improved filling as the working fluid enters the pumping chamber a point wherein the pressure of the working fluid which had already entered the pumping chamber is less than the higher pressure working fluid adjacent the outer diameter of the rotor.
Other, more recent, approaches have involved lengthening the inlet port in the direction of rotation of the rotor set adjacent the outer radial portion, the inner radial portion or both, of the pumping chambers. However, these solutions also provide less than the desired level of filling efficiency.
U.S. Pat. No. 6,896,500 to Ike et al. teaches decreasing the depth of the inlet port such that it is relatively shallow just before the pumping chambers close, apparently in an effort to direct working fluid into the pumping chamber to better fill it.
Despite the teachings of MacLeod and others, gear pumps still suffer from undesirable cavitation and operating noise due to inefficiencies in filling the pumping chambers.